Discharge lamp

ABSTRACT

A discharge lamp may include a substantially ellipsoidal discharge vessel that surrounds an anode and a cathode that are respectively fixed by current-carrying electrode holders, the latter being guided through bulb shafts arranged diametrically on the discharge vessel, there being provided around the electrode holders at the transition from the discharge vessel to the bulb shafts constrictions that form a connecting channel between the discharge space, surrounded by the discharge vessel, and in each case the bulb shaft spaces surrounded by the bulb shafts, wherein at least one of the discharge vessel, the constrictions and the anode coating is designed in such a way as to reduce or avoid blackening of the discharge vessel in the light-emitting region.

TECHNICAL FIELD

The invention relates to a discharge lamp in accordance with thepreamble of patent claim 1.

PRIOR ART

Discharge lamps, in particular XBO® high pressure discharge lamps, havean ellipsoidal lamp bulb that surrounds an anode and a cathode. Theservice life of such discharge lamps is determined, inter alia, by theblackening of the lamp bulb that occurs during operation and leads to asubstantial loss in useful light. The blackening has various causes. Oneof them is the evaporation of anode material on the basis of the hightemperatures during operation of the high pressure discharge lamp, saidmaterial being deposited on the inner surface of the lamp bulb. Afurther cause of the blackening are contaminations of the gas fill inthe lamp bulb, for example atmospheric residues such as oxygen andmoisture that can be removed only with a high outlay on time and costduring the production of the high pressure discharge lamp.

There have been various approaches to date for minimizing theblackening. For example, use is made of relatively large lamp bulbs suchthat deposits can be distributed over a relatively large area, theblackening continuing to occur in weakened form, nevertheless. A furtherapproach to this solution is to use large volume anodes in order tolower the anode temperature during operation by means of a largeemission area, and thus to reduce the evaporation of anode material.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide a high pressuredischarge lamp that has a long service life in conjunction with asubstantially maintained light intensity.

This object is achieved by a discharge lamp having the features ofpatent claim 1.

Particularly advantageous refinements are to be found in the dependentclaims.

The inventive discharge lamp has a substantially ellipsoidal dischargevessel that surrounds an anode and a cathode that are respectively fixedby current-carrying electrode holders, the latter being guided throughbulb shafts arranged diametrically on the discharge vessel, there beingprovided around the electrode holders at the transition from thedischarge vessel to the bulb shafts constrictions that form a connectingchannel between the discharge space, surrounded by the discharge vessel,and in each case the bulb shaft spaces surrounded by the bulb shafts, inwhich case the discharge vessel, the constrictions and/or the anodecoating are designed in such a way as to reduce or avoid blackening ofthe discharge vessel in the light-emitting region. This has theadvantage that each individual one of these measures substantiallyincreases the service life of a discharge lamp by comparison with theprior art, in conjunction with production costs that are approximatelymaintained.

The discharge vessel preferably has a cylindrical cooling sectionsubstantially between a side of the anode that is averted from thecathode, and a constriction. This has the advantage that, for example,evaporated anode material can accumulate in this region and that thedischarge vessel is therefore blackened outside the optically usefulregion.

The cylindrical cooling section can advantageously have a diameter thatis greater than the diameter of the cylindrical anode, and can have alength that corresponds substantially to half the length of the anode,thus enabling a sufficiently large cooling section for the depositionof, for example, evaporated anode material.

In a preferred embodiment, the anode is coated with a coating,preferably with a tungsten paste, that improves the emission. This hasthe advantage that the emission of the discharge lamp is increased,while the anode has a lower temperature and can therefore evaporate lessanode material.

The connecting channels can be embodied in such a way that they ensurethe relative position of the electrode holders in the case of minimumexhaust resistance in the production process, thus enabling a simplerand more cost effective pumping off of atmospheric residues.

The diameter and/or the length of the connecting channel can beminimized in order advantageously to achieve the minimum exhaustresistance.

Walls can be positioned obliquely with reference to the electrodeholders in the transition region between the constrictions and the bulbshafts and in the transition region between the constrictions and thedischarge vessel.

The discharge vessel has, for example, a cylindrical sectionapproximately between a side, averted from the anode, of the cathode andthe constriction, it thereby being possible to effect a mechanicallystable transition from the discharge vessel to the constriction.

An exhaust tube is preferably formed on the cooling section.

BRIEF DESCRIPTION OF THE DRAWINGS

The aim below is to explain the invention in more detail with the aid ofan exemplary embodiment. The FIGURE shows a longitudinal section througha discharge lamp in accordance with an exemplary embodiment.

PREFERRED EMBODIMENT OF THE INVENTION

The invention is explained below with the aid of an XBO® high pressuredischarge lamp that is used, for example, in projection systems andspotlights.

The FIGURE shows a schematic of an XBO® high pressure discharge lamp 1with a base at both ends using short-arc technology.

Said lamp has a discharge vessel 4 made from quartz glass with adischarge space 6 and two sealed bulb shafts 8, 10 arrangeddiametrically on the discharge vessel 4 and whose free end sections canbe provided respectively with a base sleeve (not illustrated). Twoelectrodes 14, 16 running in the bulb shafts 8, 10 and between which agas discharge occurs during operation of the lamp protrude into thedischarge space 6. Enclosed in the discharge space 6 of the dischargevessel 4 is an ionizable fill that substantially consists of high purityxenon. In the illustrated exemplary embodiment, the electrodes 14, 16are respectively embodied as a bipartite electrode system with acurrent-carrying, rod-shaped electrode holder 18, 20 and adischarge-side head electrode 22 (anode) or head electrode 24 (cathode)soldered to said holder. In accordance with the FIGURE, the right-handelectrode head 24 is embodied as a conical head cathode 24 or cathodefor generating high temperatures in order to ensure a defined arcattachment and a sufficient electron flux on the basis of thermalemission and field emission (Richardson equation).

The left-hand electrode head 22 in the FIGURE is embodied as abarrel-shaped head anode 22 or anode subjected to a high thermal load,in the case of which the emission power is improved by sufficientdimensioning of the electrode size. In order to further increase theemission power, the surface of the head anode 22 is coated with acoating 25, preferably with a tungsten paste, as a result of which thehead anode 22 has a higher emission coefficient of 0.55, and here anemission higher by approximately 40% by comparison with the prior art,where the emission coefficient is 0.4.

The rod-shaped electrode holders 18, 20 respectively have two bearingpoints. Here, one bearing point is respectively a current leadthroughsystem 26, 28 formed on the ends of the bulb shafts 8, 10, and the otherbearing point is respectively a constriction 30, 32 arranged in thetransition region between the discharge vessel 4 and the bulb shafts 8,10. The current leadthrough systems 26, 28 support the electrode holders18, 20 respectively in the radial and axial direction and are sealed inan airtight fashion against the environment such that no air can intrudefrom outside into the bulb shaft spaces 34, 36 surrounded by the bulbshafts 8, 10. Said bulb shaft spaces are connected to the dischargespace 6 of the discharge vessel 4 via connecting channels 38, 40 thatare delimited by a cylindrical inner wall 42, 44 of the constrictions30, 32 and the electrode holders 18, 20. The radial height of theconnecting channels 38, 40, which is measured from the inner walls 42,44 up to the surfaces of the electrode holders 18, 20, amounts onaverage to approximately 0.4 to 0.5 mm and is substantially higher thanin the prior art, where this height corresponds to 0.1 to 0.2 mm. Theaxial length of the connecting channels 38, 40 amounts approximately to1.5 times the cross section of the electrode holders 18, 20.

The constrictions 30, 32 have the same wall thickness as the bulb shafts8, 10, and are delimited by obliquely positioned walls 46 in thetransition region to the discharge vessel 4 and to the bulb shafts 8,10. The axial length of the constrictions 30, 32 is minimized, and theradial height of the connecting channels 38, 40 is maximized such thatthese dimensions are precisely sufficient to ensure the radial positionof the electrode holders 18, 20.

Approximately between the shadow side 48, averted from the head cathode24, of the head anode 22, and the wall 46, the right-hand one in theFIGURE, of the left-hand constriction 30, the discharge vessel 4 has asubstantially cylindrical cooling section 50 whose diameter is somewhatlarger than the diameter of the head anode 22, and whose axial lengthcorresponds approximately to half the axial length of the head anode 22.In the FIGURE, there is arranged radially on the outer periphery of thecooling section 50 an exhaust channel 52 that is used during theproduction process—described further below—of the high pressuredischarge lamp 1, and can be removed after production. A furthercylindrical section 54 is formed on the end, opposite the coolingsection 50, of the discharge vessel 4 and has a substantially shorteraxial length.

The high pressure discharge lamp 1 has an optical useful region 55 thatis marked by four dashed and dotted lines, the light being substantiallyemitted over this useful region 55 during operation.

In the prior art, after a certain period of operation during the use ofa high pressure discharge lamp blackening occurs on the inner wall ofthe discharge vessel and becomes thicker and darker as the period ofoperation lengthens. Here, this blackening is located in an opticallyuseful region and therefore reduces the useful light of the highpressure discharge lamp until the latter can no longer be used. Onecause of the blackening are the high temperatures of the anode duringoperation of the lamp, which lead to an evaporation of the anodematerial, which is then deposited on the inner wall of the dischargevessel. A further cause are contaminations of the fill of the dischargevessel with, for example, oxygen and moisture, which contaminations canlikewise be deposited in the form of a blackening.

In the case of the inventive high pressure discharge lamp 1 in theFIGURE, by contrast with the prior art a blackening 56 advantageouslybears outside the optical useful region 55 substantially against thebulb inner surface 58 of the discharge vessel 4 in the region of thecooling section 50, in the transition region between the cooling section50 and the remainder of the discharge vessel 4, and against the wall 46between the cooling section 50 and the constriction 30, this beingindicated in the FIGURE by a black coloring of the discharge vessel 4.Moreover, by comparison with the prior art the blackening 56 issubstantially lower for the same period of operation. The reasons forthis are explained below.

During production of the high pressure discharge lamp 1, gas stillpresent in the discharge vessel 4, for example air, is exhausted as faras possible from the discharge space 6 via the exhaust channel 52, andfrom the bulb shaft spaces 34, 36 via the connecting channels 38, 40.Subsequently, the discharge vessel 4 is filled with an ionizable filland sealed in an airtight fashion. Owing to their dimensioning, theconnecting channels 38, 40 here exert the highest exhaust resistance inthe high pressure discharge lamp 1. For this reason, the connectingchannels 38, 40 are dimensioned so as to have maximum height withminimum axial length, in order to minimize the exhaust resistance, asufficient radial support still being ensured for the electrode holder18, 20. On the one hand, by comparison with the prior art this enablesthe high pressure discharge lamp 1 to be evacuated in a shorter time andan exhaust resistance up to 10 times smaller, and thus permits theproduction costs to be reduced and, on the other hand, permits the airresidues such as oxygen and moisture to be minimized, since a largerquantity of air can be exhausted. The quality of the ionizable fill isimproved as a result. Smaller quantities of air residues then lead to alesser blackening 56 of the discharge vessel 4 during operation of thehigh pressure discharge lamp 1.

Owing to the higher emission of the head anodes 22 coated with tungsten,said anodes are at a lower temperature, as a result of which less anodematerial is evaporated, and thus the blackening 56 is likewise less.Furthermore, owing to the higher emission, inter alia the optical usefulregion 55 between the head anode 22 and discharge vessel 4 is heated upmore strongly than in the prior art. The cooling section 50 of thedischarge vessel 4 is shaded in by the head anode 22, and so thetemperature is lower in this region than in the remainder of thedischarge vessel 4. Evaporated anode material and contaminations of thefill are deposited in this cooling section 50 and lead to the blackening56, which is situated outside the optical useful region 55.

Owing to the abovedescribed inventive features, no blackening 56, oronly a slight one, occurs in the optical useful region 55, the resultbeing the lengthening of the service life of the high pressure dischargelamp 1 by up to 50% by comparison with the prior art.

What is disclosed is a discharge lamp having a substantially ellipsoidaldischarge vessel that surrounds an anode and a cathode that arerespectively fixed by current-carrying electrode holders, the latterbeing respectively guided through bulb shafts arranged diametrically onthe discharge vessel. Formed around the electrode holders at thetransition from the discharge vessel to the bulb shafts areconstrictions that have a connecting channel between the dischargespace, surrounded by the discharge vessel, and in each case the bulbshaft space surrounded by the bulb shafts. The discharge vessel, theconstrictions and/or the anode coating are designed in such a way as toreduce or avoid blackening in the optical useful region of the dischargelamp.

1. A discharge lamp, comprising: a substantially ellipsoidal dischargevessel that surrounds an anode and a cathode that are respectively fixedby current-carrying electrode holders, the latter being guided throughbulb shafts arranged diametrically on the discharge vessel, there beingprovided around the electrode holders at the transition from thedischarge vessel to the bulb shafts constrictions that form a connectingchannel between the discharge space, surrounded by the discharge vessel,and in each case the bulb shaft spaces surrounded by the bulb shaftswherein at least one of the discharge vessel, the constrictions and theanode coating is designed in such a way as to reduce or avoid blackeningof the discharge vessel in the light-emitting region.
 2. The dischargelamp as claimed in claim 1, wherein the discharge vessel has acylindrical cooling section substantially between a side of the anodethat is averted from the cathode, and a constriction.
 3. The dischargelamp as claimed in claim 2, wherein the cylindrical cooling section hasa diameter that is greater than the diameter of the cylindrical anode.4. The discharge lamp as claimed in claim 2, wherein the cylindricalcooling section has a length that corresponds substantially to half thelength of the anode.
 5. The discharge lamp as claimed in claim 1,wherein the anode is coated with a coating that improves the emission.6. The discharge lamp as claimed in claim 1, wherein the connectingchannels are configured in such a way that they ensure the relativeposition of the electrode holder in the case of minimum exhaustresistance in the production process.
 7. The discharge lamp as claimedin claim 6, wherein at least one of the diameter and the length of theconnecting channels are minimized.
 8. The discharge lamp as claimed inclaim 1, wherein walls are positioned obliquely with reference to theelectrode holders in the transition region between the constrictions andthe bulb shafts and in the transition region between the constrictionsand the discharge vessel.
 9. The discharge lamp as claimed in claim 1,wherein the discharge vessel has a cylindrical section substantiallybetween a side, averted from the anode, of the cathode and theconstriction.
 10. The discharge lamp as claimed in claim 2, wherein anexhaust channel is formed on the cooling section.
 11. The discharge lampas claimed in claim 5, wherein the anode is coated with a tungsten pastethat improves the emission.